Field of the Invention
The present invention relates to a motor control device.
Related Art
A control device for a brushless DC motor (abbreviated below as a “motor”) calculates the actual rotation speed of a rotor based on the magnetic field of permanent magnets that configure the rotor of a motor, detected using a the hall sensor or the like. By feeding back the detected actual rotation speed using a Proportional-Integral Controller (PI Control) or the like, motor rotation control is performed in consideration of the actual rotation speed.
Further, there are various configurations for the rotors provided at a motor, such as a two-, four-, six-, eight- or ten-pole configuration, according to the number of poles of the permanent magnets and, in general, a larger number of poles results in smoother motor rotation. As one example, a blower motor used for blowing in a vehicle air conditioning unit uses a ten-pole rotor configured of five S-poles and five N-poles.
In a motor provided with a ten-pole rotor, the S-pole magnetic field and the N-pole magnetic field are each detected five times during one rotation of the rotor. By using the magnetic fields of the S-poles and N-poles that are detected a total of thirty times by the hall sensor U for the U phase, the hall sensor V for the V phase and the hall sensor W for the W phase, detected per 60° in terms of the electrical angle, during one rotation of the rotor, the rotation of the rotor can be detected in small steps and calculation of the rotation speed of the rotor during low-speed rotation is facilitated.
However, when the rotation speed of the rotor is a high speed, the cycles for detecting the magnetic field are reduced in time, so inaccuracies in rotation speed calculation may occur as a result of errors in magnetic field detection. Further, there has been a problem, when the above-described PI control is performed using a rotation speed that has been inaccurately calculated, there is a tendency for hunting to occur, whereby motor rotation becomes irregular. Errors in magnetic field detection originate, for example, from errors in assembly of the the hall sensor U, the hall sensor V and the hall sensor W, and the influence thereof in rotation speed calculation becomes larger as the rotor rotation speed increases.
U.S. Pat. No. 5,194,545 (Patent Document No. 1) discloses a control device for a brushless motor that calculates the rotation speed of a rotor during low-speed rotation based on changes in magnetic field per 60° of electrical angle and that also calculates the rotation speed of the rotor during high-speed rotation based on changes in magnetic field per 360° of electrical angle.
However, the control device for a brushless motor described in Patent Document No. 1 does not calculate the rotation speed at an electrical angle between per 60° of electrical angle and per 360° of electrical angle. As a result, calculation of the rotation speed when the rotation speed of the rotor transitions from a low speed to a high speed becomes inaccurate, and there has been the problem whereby there is a tendency for the above-described hunting to occur in the motor rotation.